alpha-Thrombin induced a change in the cell morphology of IIC9 fibroblasts from a semiround to an elongated form, accompanied by an increase in stress fibers. Incubation of the cells with phospholipase D (PLD) from Streptomyces chromofuscus and exogenous phosphatidic acid (PA) caused similar morphological changes, whereas platelet-derived growth factor (PDGF) and phorbol 12-myristate 13-acetate (PMA) induced different changes, e.g., disruption of stress fibers and cell rounding. alpha-Thrombin, PDGF, and exogenous PLD increased PA by 20-40%, and PMA produced a smaller increase. alpha-Thrombin and exogenous PLD produced rapid increases in the amount of filamentous actin (F-actin) that were sustained for at least 60 min. However, PDGF produced a transient increase of F-actin at 1 min and PMA caused no significant change. Dioctanoylglycerol was ineffective except at 50 micrograms/ml. Phospholipase C from Bacillus cereus, which increased diacylglycerol (DAG) but not PA, did not change F-actin content. Down-regulation of protein kinase C (PKC) did not block actin polymerization induced by alpha-thrombin. H-7 was also ineffective. Exogenous PA activated actin polymerization with a significant effect at 0.01 microgram/ml and a maximal increase at 1 microgram/ml. No other phospholipids tested, including polyphosphoinositides, significantly activated actin polymerization. PDGF partially inhibited PA-induced actin polymerization after an initial increase at 1 min. PMA completely or largely blocked actin polymerization induced by PA or PLD. These results show that PC-derived PA, but not DAG or PKC, activates actin polymerization in IIC9 fibroblasts, and indicate that PDGF and PMA have inhibitory effects on PA-induced actin polymerization.

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